1. Field of the Invention
The present invention relates to a pyrometer, more particularly, it relates to a pyrometer for measuring the temperature of a target by employing light as a medium.
2. Description of the Prior Art
In such a pyrometer, it is necessary to know the emissivity of a target to be measured, but in general, such emissivity can not be known. Therefore, in a plurality of proposed pyrometers, the emissivities of a target to be measured are assumed as follows: In Japanese Laid-Open Patent Application No. 130622/1981, light having two different colors (two different wavelengths .lambda..sub.1, .lambda..sub.2) is measured, and the emissivity .epsilon. in the wavelength .lambda..sub.1 is assumed to be equal to the emissivity .epsilon. in the wavelength .lambda..sub.2. In the other Japanese Laid-Open Patent Application No. 130623/1981, light having three different colors (three different wavelengths) is measured, and the emissivities .epsilon. (.lambda.) in respective wavelengths are assumed that .epsilon. (.lambda.)=exp.(a.sub.0 +a.sub.1 .lambda.), wherein a.sub.0 and a.sub.1 represent constants respectively. Furthermore, in the U.S. Pat. No. 4,411,519, three temperatures T.sub.12, T.sub.23, and T.sub.31 are calculated with assuming .epsilon. (.lambda..sub.1)=.epsilon. (.lambda..sub.2), .epsilon. (.lambda..sub.2)=.epsilon. (.lambda..sub.3), .epsilon. (.lambda..sub.3)=.epsilon. (.lambda..sub.1) respectively, and a true temperature T is obtained by T=(T.sub.12 +T.sub.23 +T.sub.31)/3.
Meanwhile, in the other Japanese Laid-Open Patent Application No. 30727/1986, the intensities L(.lambda..sub.1) and L(.lambda..sub.2) of light reflected by the target are measured in two different wavelengths .lambda..sub.1, .lambda..sub.2 respectively, and the temperature is calculated on the basis of the measured light intensities L (.lambda..sub.1), L (.lambda..sub.2), and a reflected light intensity ratio L(.lambda..sub.1)/L(.lambda..sub.2) represented by the emissivities .epsilon. (.lambda..sub.1), .epsilon. (.lambda..sub.2) as follows: EQU L(.lambda..sub.1)/L(.lambda..sub.2)={1-.epsilon.(.lambda..sub.1)}/{1-.epsil on.(.lambda..sub.2)} (A)
Namely, assuming that the measured radiation intensity in wavelength .lambda. is D(.lambda.) and the radiation intensity of blackbody in the wavelength .lambda. at temperature T is D.sub.0 (.lambda., T), the following equation is established: EQU .epsilon.(.lambda.)=D(.lambda.)/D.sub.0 (.lambda., T) (B)
Here, the temperature T can be calculated in accordance with the equation (A), since the unknown factor is only T in the equation (A) if .epsilon. (.lambda.) shown in the equation (B) is substituted to the equation (A). D.sub.0 (.lambda., T) can be calculated in accordance with the well-known Planck formula and constants peculiar to the device.
However, in Japanese Laid-Open Patent Application Nos. 130622/1981 and 7529/1982, since it is assumed that .epsilon. (.lambda..sub.1)=.epsilon. (.lambda..sub.2), the true temperature can be calculated only when a spectral characteristic of emissivity is constant. In other words, when the spectral characteristic of emissivity is not constant, assumption .epsilon. (.lambda..sub.1)=.epsilon. (.lambda..sub.2) itself is erroneous, so that the true temperature can not be calculated. In either case, in Japanese Laid-Open Patent Application No. 130622/1981, U.S. Pat. No. 4,411,519 and including Japanese Laid-Open Patent Application No. 130623/1981, since information on the reflected light is not included in the assumption of an emissivity, the true temperature can not be obtained except in the case of constant spectral characteristic of emissivity.
On the other hand, in the calculation in Japanese Laid-Open Patent Application No. 30727/1986, there may be the case that two different temperatures are obtained. This is because that, the equation R(.lambda..sub.1)/R(.lambda..sub.2)={1-.epsilon. (.lambda..sub.1)}/{1-.epsilon. (.lambda..sub.2)}, where R is a reflectance, used in the calculation is only established when all reflected light from the target to be measured are collected or it has a perfect diffusion surface.
Therefore, in a preceding U.S. patent application of Ser. No. 203,003, it is suggested to provide a pyrometer which is possible to calculate the true temperature even when the spectral characteristic of emissivity is not constant, by including measured reflection information.
However, a pyrometer disclosed in the aforesaid U.S. Patent application performs temperature measurement of a target on the premise that the target is a non-transparent object and is not directed to temperature measurement for a semi-transparent object. However, besides non-transparent objects, there are also present semi-transparent objects such as a semi-conductor wafer, ceramic (ceramic has a semi-transparent characteristic in the infrared region) and the like among objects whose temperatures are necessary to be measured. Accordingly, it has been desired to realize a pyrometer which is possible to calculate the temperature of such a semi-transparent object. And in this case, the emissivity of the semi-transparent object is necessary for temperature calculation.